This invention relates generally to electronic circuits and more particularly to a method and system for generating multiple bias currents.
Many electronic components include a plurality of devices that require a bias current for optimum operation. Such devices include, for example, low noise amplifiers, filters, mixers, and automatic gain controls. All of these devices may be used in, for example, a wireless phone. The bias current is used to optimize linearity, gain, and noise characteristics of such devices. The bias currents typically vary in a fairly linear fashion with some parameter, such as temperature. Because such currents vary in a fairly linear fashion, they are often referred to as current slopes. These current slopes are provided by a current slope generator. An adjustment circuit is often used with a current slope generator to modify the nominal value of current supplied; however, the slope of the current remains the same regardless of the adjustment circuit.
Because an electronic component may include a plurality of devices requiring biasing, it is often desirable to have a plurality of current slopes available for such an electronic component. However, because each device receiving a bias current may have different operating parameters, it is often desirable to provide bias currents having different slope characteristics for each device.
Current slope generators may be classified into two categories: (1) a single current slope generator, and (2) a multiple current slope generator. Each type of slope generator generally produces a current (or a plurality of currents) that is proportional to a resistance element associated with the current generator. In some instances, the current may be proportional to a single resistor; in others, the current may be proportional to a combination of resistance elements. These resistance elements generally have a tolerancexe2x80x94which means the actual magnitude of resistance may vary with individual resistors. Because the actual value of resistance may vary, and the generated current is proportional to the resistance, the currents generated by current slope generators will have a tolerance equal to that of the associated resistors in the current slope generator.
It is often desirable, however, to provide bias currents having tolerances greater than that available from current slope generators. Therefore, enhancement circuits are utilized. For single slope current generators, an enhancement circuit utilizing a precision resistor is used. Use of this circuit generates a current that has a tolerance equal to that of the precision resistor. In such a system, an external precision resistor is applied to a chip incorporating the current slope generator and the adjustment circuit to provide the increased accuracy. A problem with this type of system, however, is that a single slope generator can produce currents having only a single slope, which is sometimes unacceptable. Multiple slope current generators may also be enhanced. However, according to conventional techniques, a precision resistor is required for each different current slope. In many applications, only one pin is available for receiving a precision resistor. Therefore, it is often impractical to provide a plurality of different current slopes in conjunction with more precise tolerances.
Accordingly, a need has arisen for an improved method and system for generating multiple bias currents. The present invention provides a system and method for generating multiple bias currents that addresses shortcomings of prior systems and methods.
According to one embodiment of the invention, a method for generating a plurality of enhanced accuracy current slopes includes providing a plurality of current slopes and summing signals indicative of each of the plurality of current slopes to generate a current slope sum. The method also includes generating an enhanced accuracy current slope sum based on the current slope sum and generating the plurality of enhanced accuracy current slopes based on the enhanced accuracy current slope sum such that each respective ratio between each enhanced accuracy current slope and the enhanced accuracy current slope sum is approximately equal to each respective ratio between each signal indicative of the corresponding current slope and the current slope sum.
According to another embodiment of the invention, a system for generating a bias current includes a current slope generator operable to generate a plurality of currents proportional to a resistance value. The resistance value has a tolerance. The system also includes a current adjustor operable to generate a plurality of enhanced accuracy currents bias currents. The current adjustor includes a current transfer circuit operable to receive the plurality of currents and in response generate a current sum. The current adjustor also includes an accuracy enhancer circuit for receiving the current sum and for connection to an enhanced accuracy resistance element. The accuracy enhancer circuit is operable in combination with the enhanced accuracy resistance element to generate an enhanced accuracy current sum through the enhanced accuracy resistance element. The current adjustor also includes a differential voltage isolator for receiving, from the current transfer circuit, at least one differential voltage associated with at least two of the plurality of currents. The differential voltage isolator is operable to generate, based on the enhanced accuracy current sum, a plurality of enhanced accuracy currents corresponding to respective ones of the plurality of currents, each enhanced accuracy current being a proportion of the enhanced accuracy current sum that is approximately equal to the proportion that the corresponding current is of the current sum. The current adjustor also includes a current slope applier operable to receive the signals indicative of the plurality of enhanced accuracy currents and generate corresponding currents for use as the bias currents.
Embodiments of the invention provide numerous technical advantages. For example, in one embodiment of the invention, a method is provided for generating a plurality of bias currents having different slopes, but also having increased accuracy. Thus, a single chip having a plurality of devices that require biasing may utilize different biasing slopes for each device while maintaining a high level of accuracy for such biasing. These advantages are utilized while, at the same time, utilizing only one pin on a chip for a single precision resistor.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.